1. Field of the Invention
This invention relates to an automatic transmission installed in an automobile, and more particularly to a control apparatus of a multistage automatic transmission comprising a main shifting unit and a sub-shifting unit.
2. Description of the Related Art
Generally, an automatic transmission includes a torque converter and a planetary gear shifting mechanism. The gear shifting mechanism comprises an overdrive (O/D) planetary gear unit, a front planetary gear unit and a rear planetary gear unit. The gear shifting mechanism offers four forward stages and one rear stage by means of two solenoid valves and three shifting valves.
Also, as disclosed in a Japanese Laid-open Patent Application No. 57-37140, there has been proposed by the present applicant an automatic shift control apparatus for obtaining six forward stages of shifting, wherein three solenoid valves and three shifting valves are provided, an overdrive planetary gear unit functioning as a sub-shifting unit is used in combination with a main shifting unit comprising a front and a rear planetary gear unit.
By the way, the multistage automatic transmission comprising the sub-shifting unit and main shifting unit obtains 6 stages of shifting through a combination of two stages obtained from the overdrive and direct connection of the sub-shifting unit and first speed, second speed and third speed of the main shifting unit. At this time, there occurs a situation in which one of the shifting units is down-shifted while the other is up-shifted. For example, the sub-shifting unit is down-shifted from the overdrive state to the directly connecting state and simultaneously the main shifting unit is up-shifted from first speed to second speed, thus shifting from second speed to third speed in total. In the foregoing case, according to the conventional control apparatus, since no countermeasure is taken to complete both the shifting operations at the same time, there is a possibility that one of the shifting operations is effected first and then the other shifting operation is effected. For example, when the down-shift is effected first, the shifting unit is first reduced (to first speed) in speed extensively, and thereafter increased (third speed) in speed extensively. On the other hand, when the up-shift is effected first, the shifting unit is first increased (to fourth speed) in speed extensively and thereafter reduced in speed. Due to the foregoing, a blow-up and fall in engine speed occurs and so does a large shifting shock.